Hi Andrew,
I'm not that sure but there seem to be a building:levels:aboveground
besides the building:levels tag. So levels-aboveground=levels below ground?
http://wiki.openstreetmap.org/wiki/OSM-3D#Buildings
Currently we put all 3D "experts" together to make a better roadmap on
what and how to use 3D datas in OSM, feel free to join.
http://wiki.openstreetmap.org/wiki/3D_Development
regards
Matthias
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